The present invention relates to a hydraulic brake system for automotive vehicles of the type comprising a directly operable master brake cylinder, at least one wheel brake in communication with the master brake cylinder, a motor-driven pump capable of being connected to the wheel brake, a brake valve operable by the pressure in the master brake cylinder for controlling the pump pressure, and a brake skid control means comprising a valve arrangement connected ahead of the wheel brake, through which the wheel brake can, alternatingly, be connected to the pump and to a non-pressurized reservoir.
In brake systems of the above-described type, the brake pressure for operating the wheel brakes can be generated through both the master brake cylinder and through a motor-driven pump. The pump pressure is hydraulically controllable in response to the operating pressure in the master brake cylinder. These brake systems are relatively simple in construction as no accumulator or mechanical transmission means between the brake pedal and the brake valve are needed.
In a brake system known from W. German application P No. 35 38 284.8, individual wheel brakes comprise a first and second hydraulic clamping unit. The controlled pump pressure is exclusively applied to the second clamping unit while pressure from a pressure chamber of the master brake cylinder is exclusively applied to the first clamping unit of the wheel brakes. For brake skid controlling, the wheel brakes comprise a hydraulic releasing unit to which the pump pressure is applied by the valve arrangement, in controlled form, to counteract the clamping units. By so dividing the wheel brake applying means into two clamping units and one releasing means, a high operating safety is provided because the brake, upon failure of the energy supply or of the brake skid control, will remain operable through the second clamping unit with the aid of the master brake cylinder. The clamping units and the releasing means on the wheel brakes are piston cylinder systems that are complex in construction and involve relatively high space requirements.
In another brake system of the type described in W. German Patent Application P No. 35 38 284.8, the pressure fluid delivered by a pump is conducted only to partial circuits of the brake system. Hydraulically driven switch-over valves are provided which, upon introduction of the pump pressure, separate the partial circuits from the master brake cylinder and the remaining partial circuits of the brake system. The partial circuits capable of being connected to the pump, also in this brake system, may be in communication with a brake skid control system. The required safety against failure of the energy supply through the pump, or of the brake skid control means, in that brake system, is safeguarded by the partial circuits that are operable only with the aid of the master brake cylinder. This type of brake system exhibits the disadvantage that only the partial circuits to be connected to the pump can be furnished with a brake skid control. Moreover, the operating reliability, in that brake system, to a high degree, is dependent on the proper switching of the switch-over valves.